1. Field of the Invention
The present invention relates to a method, device and system for providing broadband radio access with a high degree of coverage and a high system capacity even in congested areas where obstacles to line-of-sight communications are present. More particularly, the present invention relates to a method, device and system for broadband radio access that includes a millimeterwave (ML) system segment integrated with a microwave (MK) system segment.
2. Discussion of the Background
Local Multipoint Distribution Service (hereinafter LMDS), Multipoint Video Distribution System (hereinafter MVDS) and High Capacity Point to Multipoint (hereinafter HPMP) are examples of ML systems, while Multipoint Microwave Distribution System (hereinafter MMDS) and terrestrial-based television (hereinafter terrestrial TV) systems are examples of microwave (MK) systems. ML systems operate as fixed point to multipoint radio systems at frequencies over 10 GHz. By having fixed high frequency systems over 10 GHz makes it possible for simple construction of cellular systems where very good frequency utilization factors (less than or equal to 1) can be achieved. Good frequency utilization allows for a larger amount of information to be transmitted over a predetermined bandwidth. Furthermore, ML systems also have the opportunity to operate at available, unused frequency bands over 20 GHz, which makes it possible to use such systems to achieve excellent system capacity that support broadband access for many subscribers.
As recognized by the present inventors, a problem with using frequencies above 10 GHz, is that the radiowave propagation between terminals is primarily achieved through line-of-sight (hereinafter LOS) communications. The communication channel at this frequency is obstructed with physical structures, which blocks the radiowaves. Such obstacles include buildings, trees, and foliage, or even terrain obstacles such as hills and naturally occurring obstructions. This LOS requirement has been verified at these frequencies by initial propagation measurements taken at Telia Research, as well as from Bellcore and Texas Instruments, for example. Developed urban areas as well as residential areas pose a problem for LOS communications, particulary for subscribers in low buildings surrounded by higher surrounding vegetation and buildings. Generally, even though the broadcast antenna from the ML base station may be elevated, the communication range is limited to less than about 5 km from the base station due to, for example, high propagation attenuation and attenuation by rain, or any other channel obstructions.
MK stems operate under 10 GHz and are not as restricted by the line-of-sight channel requirement as is the case for ML communication systems. Accordingly, it is possible to employ MK base stations having a base station antenna (BSA) and a subscriber's terminal antenna (ATM) being located closer to the ground than for ML systems, and also with obstructions placed therebetween. This is the case, because radio frequencies under 10 GHz are generally capable of penetrating the obstructions to some extent, and have diffraction properties that are more favorable than at the higher frequencies. Accordingly, from a service provider perspective, the number of subscribers who could be covered by an MK system increases thereby obtaining greater potential subscribership for a predetermined geographical area.
However, it is recognized by the present inventors that technical problems associated with MK systems is that at the lower frequencies the electromagnetic energy propagates better, and therefore a lower frequency repeating factor (reuse rate) results in a lower system capacity. Accordingly, even though a large number of subscribers may be covered by a single transmitter, only a small portion of the subscribers can actually use the MK system, due to the lowered capacity of the MK system and its associated limited frequency repeating factor. While it is possible to use higher level modulation techniques to provide a greater amount of information per modulation symbol, the higher modulation level does not actually increase, in a significant way, the system capacity because larger signal/noise ratios (S/N) are required to support the more advanced modulation schemes. Consequently, the total system capacity for cellular applications will not necessarily increase with MK-systems that use a higher modulation level.
A description of such modulation techniques, as well as other background art devices and methods associated with the above described communication concepts and the present invention, may be found in Sklar, B., "Digital Communications Fundamentals and Applications", Prentice Hall, ISBN 0-13-211939-0, 1998. For wireless cable and other communication systems and components that operate above 1 GHz, a discussion is found at Berkoff, S., et al., "Wireless Cable and SMATV", Baylin Publications, ISBN 0-917893-17-4, 1992. The entire contents of these two references, being incorporated herein by reference. For convenience, a summary of features and attributes of MK systems and ML systems, is provided in Table 1, which follows.
TABLE 1 Frequency comparison for fixed, broadband radio access. The comparison is comprehensive and describes typical differences rather than absolute data. Frequency MK-system ML-system Features &lt;10 GHz &gt;20 GHz Range: high medium (typically &lt;50 km) (typically &lt;5 km) Frequency repetition: low-medium good Modulation: 4-6 bit per symbol 2 bit per symbol Demand for line-of-sight: medium high Supply of bandwidth: low-medium high Uplink: low capacity medium-high capacity Physical size, directional medium small antenna: